Carbon nanotube induced variations in structure，morphology and luminescence of YBO3：Eu3＋（5%）

ZOU Dan，MA Yong－qing

（Anhui Key Laboratory of Information Materials and Devices，School of Physics and Materials Science，Anhui University，Hefei 230039，China）

0 Introduction

In the last decades，nanostructured luminescent materials have generated technological interest since nanosized phosphors are expected to improve display resolution and other applications［1－3］.Furthermore，nanostructured luminescent materials exhibit novel properties because optical properties depend on size and morphology［4－6］.Recently，nano－sized YBO3：Eu3＋was reported to improve fluorescence intensity and color purity［7］.The charge－transfer excitation bands of Eu3＋in the nanowires and nanotubes blue－shift in contrast to those in bulk，because of the variation of coordination environments［8］.These results indicate that nano－sized YBO3：Eu3＋is promising for applications in displays and optical devices.YBO3：Eu3＋with nest－like，rose－like，cruller－like，flowerlike and cake－like morphologies［9－10］has been reported before. However，one－dimensional and dendritic－structured YBO3：Eu3＋，specially synthesized with a carbon nanotube（CNT）template，has not been reported before to our knowledge.

In the present work，the dendritic and spherical－like YBO3：Eu3＋nano－structures were synthesized by a facile hydrothermal method and subsequently annealed at 1 000℃with and without additive carbon nanotube in the precursor solution.We observed red－shift of the charge－transfer excitation bands of Eu3＋rather than blue－shift in previous report.Interestingly the sample with dendritic structure has higher red to orange ratio，and its CIE coordinate（0.67，0.33）matches better with that of commercial red phosphors Y2O2S：Eu3＋，indicating better color purity，which can be used as potential candidates for nano－devices and lighting devices.

1 Experiment

Stoichiometric Eu2O3was dissolved in HNO3.Y（NO3）3·6H2O and H3BO3were dissolved in deionized water with stirring for 10minutes to form solution，respectively.The Eu（NO3）3，Y（NO3）3and H3BO3solution was mixed with continuous stirring.The obtained solution was divided into two parts.Appropriate amount of MWNT－COOH and sodium dodecyl benzene sulfonate（SDBS）were dispersed in deionized water and sonicated until the homogeneous suspension was obtained.Then，the suspension was added to one of above two parts.Next appropriate amount of C2H5OH was added into two kinds of solution and NH3·H2O was added to adjust pH value to 8.Finally，two kinds of precursor solution were placed in a stainless－steel autoclave with a Teflon linear of 50mL capability and heated at 180℃for 12h.After the autoclave was cooled to room temperature naturally，the products were separated by centrifugation，washed with ethanol and deionized water several times，and dried at 80℃to obtain the precursor powders.The precursor powders were subsequently sintered at 1 000℃to obtain the final samples，which were hereafter labeled as YBO－CNT and YBO for those with and without additive MWNT－COOH in the precursor solution，respectively.

The crystal structure of the products was characterized by X－ray diffraction using an X－ray diffractometer（XRD；DX－2000SSC）with CuKαirradiation（λ＝1.541 8?）in the scanning range 15 to 85°with a step of 0.02°.Scanning electron microscope （SEM，S－4800，Hitachi）and （high resolution）transmission electron microscopy（（HR）TEM，JEOL JEM－2100）were used to observe morphologies microstructures.The excitation and emission spectra were measured on a FL fluorescence spectrophotometer（F－4500）.All the measurements were carried out at room temperature.

2 Results and discussion

Fig.1 shows the XRD results of samples YBO－CNT （a）and YBO （b）.For sample YBO－CNT，the positions and the relative intensities of main diffraction peaks are well matched with those of JCPDS（No.16－0277）pattern of YBO3，which has a hexagonal vaterite－type structure with space group P63／m（176）.The other diffraction peaks in sample YBO－CNT can be indexed to those of Y3BO6which has a monoclinic structure with space group C2／m（12），compared with the standard JCPDS card （No.34－0291），indicating that sample YBO－CNT contains two phases of YBO3and Y3BO6.However，sample YBO exhibits the single－phase YBO3.

TEM image of the hydrothermally prepared sample at 180℃with CNT added in the precursor solution exhibits long and uniform nanowires with diameter of about 50nm，which exhibits good dispersivity，as shown in Fig.2a.This implies that the CNTs templates essentially determine the shape of the sample.

HRTEM image in the inset of Fig.1adistinctly show the core－shell nanostructure with diameter of CNT core being about 20nm and the thickness of shell about 15nm.When the hydrothermally prepared sample was annealed at 1 000 ℃，some residual organic components and CNTs were decomposed or burned out during the calcinations process.The dispersive nanowires conglomerated to form dendritic structures with diameter of about 50nm for the sample YBO－CNT，as shown in Fig.2b.However，the hydrothermally prepared sample at 180℃ without additive CNT in the precursor solution includes some spherical particles with diameter in the range of 5－10 μm.After annealed at 1 000℃，the obtained sample YBO retains the spherical－like morphology，but the surface cracks as a result of residual organic matter decomposing.

Fig.3 ashows excitation spectra measured by monitoring the 627nm emission of Eu3＋.Fig.3b shows emission spectra measured underλex＝247nm for samples YBO－CNT and YBO.The excitation spectrum of YBO shows a weak peak at 220nm due to the charge transfer transition between Y3＋andA broad and strong excitation band peaking at 247nm results from the charge transfer transition of Eu3＋.The emission spectrum of YBO is composed of several sharp lines locating at 594（5D0－7F1），613and 627（5D0－7F2），652and 674（5D0－7F3），and 704（5D0－7F4）nm，resulting from the emission of Eu3＋due to transitions of5D0－7FJ（J＝1，2，3and 4）.The most intense red emission（R）at 627nm results from the electric dipole transition5D0－7F2，while the orange emission （O）at 594nm from5D0－7F1transition is a typical magnetic－dipole transition.The emission intensity ratio between red（627nm）and orange（594nm），i.e.R／O，is 1.08.

For sample YBO－CNT，the excitation band around 247nm becomes weak and the most intense excitation band due to CT transition in Eu3＋－O2－red－shifts to 279nm.Compared with YBO，YBO－CNT exhibits weaker emission.The orange emission at 594nm is greatly decreased，and the red emissions at 613and 627nm in YBO merge into a single and broad emission at 627nm in YBO－CNT.The broad luminescence band in the visible light range is effective to obtain a good color rendering property for the lighting devices.TheR／Oratio of YBO－CNT is 3.83，which is about three times larger than that of YBO.

While samples YBO－CNT and YBO were excited by ultraviolet light withλex＝254nm，the emission intensity of YBO－CNT becomes stronger than that of YBO （see Fig.4），indicating that YBOCNT can be more efficiently excited by ultraviolet light with longer wavelength.The emission intensity ratioR／Ois 1.30and 4.48for YBO and YBO－CNT，respectively.Based on the results in Fig.3and Fig.4，YBO－CNT exhibits higherR／Oratio than YBO whether excited by 247nm or by 254 nm，indicating that improved chromaticity can be obtained by adding CNT in the precursor solution.Insets in Fig.4show the fluorescent photos under 254nm ultraviolet light exciting，illustrating the better color－purity for YBO－CNT rather than YBO and indicating that the obtained novel nanostructured YBO－CNT can be used as potential candidates for nano－devices and lighting devices［7，11］.The emission color purity can also be expressed by the CIE coordinates （x，y）.The calculated CIE chromaticity coordinates（x，y）of samples YBO－CNT and YBO are（0.67，0.33）and（0.65，0.35），respectively.The chromaticity coordinate （0.67，0.33）of YBO－CNT is well in agreement with that（0.665，0.334）for commercial red phosphors Y2O2S：Eu3＋［12］，satisfying the requirement for red phosphors.The abnormal luminescent behavior for YBO－CNT correlates to the microstructure［13］.The smaller particle of YBO－CNT than YBO increases the effective surface area of the particles.Many atoms on the nanoparticle surface can not be bound successfully，leading to numerous defects on the surface.These defects may increase the degree of crystal field symmetry disorder and lower the local symmetry of the Eu3＋ions.Thereby the probability of the5D0－7F2redemission transition will be increased.

3 Conclusions

The hydrothermally as－prepared sample exhibits one－dimensional morphology in the case that the carbon nanotube is added to the precursor solution as a template，which changes into dendritic structure with diameter of about 50nm after annealed at 1 000℃.The annealed sample using carbon nanotube as a template exhibits good color purity，comparable to commercial red phosphors Y2O2S：Eu3＋，which can be used as potential candidates for nano－devices and lighting devices.

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